A number of techniques are known for warning against the presence of intruders, the most common of which involve the use of ultrasonic and/or passive infrared sensors. While being relatively inexpensive to make and install, these devices are prone to false alarms due to environmental changes such as temperature fluctuations and, in the case of ultrasonics-based alarms, air pressure fluctuations. In addition to increasing the likelihood of false alarms, environmental changes such as temperature variations also lead to undesirable changes in alarm sensitivity.
Microwave sensors are available, but up to now these have been relatively expensive and have produced only marginal improvement over the more common ultrasonic and infra-red types. They have also failed to meet official specifications regarding EMC (electromagnetic compatibility) and frequency stability.
A conventional Doppler sensor arrangement is shown in FIG. 1. In FIG. 1, the output of a microwave oscillator 12 is fed to a transmit antenna 14 through a coupler 13, after being first amplified, if necessary, in an amplifier 15. A return signal, which may differ from the transmitted frequency where movement has occurred within the space illuminated by the transmitted signal, is received on a receive antenna 16 and fed to a mixer 17 where it is mixed with the coupled local oscillator signal 12 at an input 11 to generate a baseband IF signal on a line 18. The IF signal on line 18 is then taken to a signal processing stage 19 which provides an indication, usually audible, that movement has taken place.
The use of two antennas and a separate mixer and oscillator add to the complexity and hence cost of the conventional arrangement. In addition, such known arrangements have, up to now, had poor frequency stability and have been prone to false alarms due to factors such as electromagnetic interference entering the alarm and the alarm being triggered by the movement of targets located outside the vehicle being monitored.